Field of the Invention
The invention relates to an electrically heatable honeycomb body, having an axis, an inner region surrounding the axis, an outer region spaced apart from the axis, and at least one stack including at least one sheet metal layer being disposed between the inner and the outer regions and being wound in the manner of a spiral, with a multiplicity of windings, around the inner region.
Such honeycomb bodies, that are made up of stacks of sheet metal layers which are wound or intertwined, are described in Published European Application No. 0 245 736 B1; Published European Application No. 0 245 737 B1, corresponding to U.S. Pat. Nos. 4,832,998 and 4,923,109; and Published International Application WO 90/03220 A1. The use of such honeycomb bodies as carriers for catalysts, especially for use in motor vehicle exhaust systems to eliminate pollutants from the exhaust gases of the internal combustion engines driving the motor vehicles, is explained in detail in those references. An especially advantageous feature of a metal honeycomb body as a carrier for a catalyst is the provision of a direct electrical heating, since a typical catalyst, for converting pollutants in engine exhaust gas, does not develop its effectiveness until temperatures above approximately 200.degree. C. so that it does not become active until a delay of possibly several minutes, after it has begun to be acted upon by exhaust gas. Heating is therefore extremely desirable in order to speed up the activation of a catalyst, especially a catalyst in a motor vehicle exhaust system. As such, in the case of a metal honeycomb body, direct electrical heating is especially attractive, in which the honeycomb body would be acted upon by electric current, for example from the battery of the motor vehicle having the exhaust system in which the honeycomb body is installed. However, depending on the model, the electrical resistance of a honeycomb body to be installed in an exhaust gas system, of the type described in the aforementioned references, is at most approximately one-hundredth of an ohm. Such a honeycomb body cannot be connected to a typical motor vehicle battery without straining the battery in an unacceptable way. In view of such problems, provisions have already been considered for increasing the electrical resistance of a honeycomb body and in particular arriving at the use in motor vehicles of honeycomb bodies that can be adequately heated within short time periods without unacceptably straining the sources of electrical energy that are typically available. Accordingly, Published International Applications WO 89/10470 A1 and WO 89/10471 A1 describe subdividing a honeycomb body with gaps and/or electrically insulating partitions in such a way that at least one electrical current path through the honeycomb body, with an electrical resistance of the desired order of magnitude, particularly approximately 0.1 Ohm, is produced. However, the solutions to the problem of electrical resistance described in the two references may possibly lead to difficulties, if the honeycomb body to be electrically heated is intended to withstand severe mechanical strains. In that case, the ceramic layers and above all the gaps that are possibly to be provided between the individual sheet-metal layers or windings mean that connection points between the sheet metal layers in the honeycomb bodies must be left out, thereby loosening its structure and possibly reducing its rigidity. The capacity of such a honeycomb body to withstand vibration and the like may be limited in comparison with otherwise identical honeycomb bodies that are not constructed for electrical heating.